In recent years, due to advances in DSL, FTTH, CATV, and other Internet technology, broadband access networks are rapidly spreading and utilization rates are becoming cheaper, so rapid progress is being made in establishing an environment for continuous connection to IP networks. For this reason, the environment is being established for enabling IP telephone and other various applications offered on IP networks to be easily utilized by numerous users.
Here, if viewed from communication networks as a whole, the changeover from communication networks based on public switched telephone networks (PSTNs) to communication networks based on IP (Internet Protocol) networks is accelerating. Regarding the former PSTN networks, various services and applications have been perfected after long years of effort. Thus, there is still a considerable difference from those offered by the latter IP networks. However, all of the various services and applications offered by PSTN networks were developed and provided under access infrastructures which are comprised of existing metal cables.
Under such a situation, operators have to move existing PSTN services etc., as they are, to the NGN while keeping the PSTN services running, that is, the so-called problem of “migration”. The technique of just providing PSTN services etc. on Voice over Internet Protocol (VoIP) is not enough to solve the “migration” problem.
In recent years, a device has proposed which can flexibly cope with demands for such migration. This is the above access gateway (AGW). This is called a “Multi Service Access Node” (MSAN) as well. The AGW is a device enabling both PSTN and NGN services to be provided cost efficiently by various types of access technology, for example POTS (Plain Old Telephones Services), ISDN (Integrated Services Digital Networks), xDSL (x Digital Subscriber Lines), and PON (Passive Optical Networks). Thus, the device provides a platform provided with the function as a gateway to an NGN core network.
The carrier introduces such an AGW into the communication network so as to be able to continuously provide the above various existing services, while smoothly shifting the communication network to the NGN, and so as to be able to provide new services or applications under the above diverse access technology.
For example, by introducing the AGW, it is possible to provide voice services under the ISDN etc. in a conventional PSTN and simultaneously use the gateway function of the AGW to connect to a soft switch (SS), provided in the NGN, so as to provide IP telephone voice services using metal cables.
Note that, as related art relating to the embodiments, there is the following Patent Document 1. The Patent Document 1 discloses a VoIP gateway device which is designed to detour a call received from an ISDN terminal, from an IP network to a detour network so as to be received at the other party, even in a case where the number system utilized in the IP network differs from the number system utilized in the detour network.    Patent Document 1: Japanese Patent Publication (A) No. 2006-229303
As explained above, in the process of the changeover from a PSTN network to an NGN IP network, the progress in converting the core network to IP and the progress in converting the subscriber side access network to IP may not be identical, when compared with each other. For example the subscriber side access network, provided with an integrated access device (IAD) under it, may be rapidly converted to IP. In an area having such an advanced IP access network, many users seek further improvement of the quality of communication. For example, the quality of communication, for which improvement is sought, is a high quality of communication equivalent to the quality of communication offered by the existing PSTN.